1. Field of the Invention
The present invention relates generally to space-time block coded orthogonal frequency division multiplexing and, more particularly, to a method and apparatus for detecting space-time block coded orthogonal frequency division multiplexing signals in time-variant channels.
2. Description of the Related Art
Recently, in order to cope with fading distortion due to a multi-path channel at the time of transmitting wireless wideband signals, research into space diversity techniques of improving link fading margin performance using a plurality of antennas at the transmitting and receiving ends of an Orthogonal Frequency Division Multiplexing (OFDM) system has been conducted.
A representative of space-time diversity techniques that are implemented at a transmitting end is Space-Time Block Coding (STBC). STBC was initially proposed as a transmit antenna diversity coding technique using two transmit antennas by Alamouti [see S. M. Alamouti, “A Simple Transmit Diversity Technique for Wireless Communications”, IEEE J. Select. Areas Comm., Vol. 16, No. 8, October, 1998; hereinafter referred to as “Reference 1”]. Thereafter, the transmit diversity coding technique could be expanded to cases involving an arbitrary number of transmit antennas based on an orthogonal design condition by Tarokh [see V. Tarokh, H. Jafarkhani, and A. R. Calderbank, “Space-Time Block Codes From Orthogonal Designs,” IEEE Trans. Inform. Theory, Vol. 45, No. 5, July, 1999; hereinafter referred to as “Reference 2”].
Meanwhile, in an STBC-OFDM system, an STBC encoder is independently applied to N sub-carriers corresponding to sub-channels that experience flat fading. At this time, the encoding method is determined according to the number of transmit antennas.
FIG. 1 is a schematic block diagram of a conventional STBC-OFDM communication system 100 having P transmit antennas and Q receive antennas.
Referring to FIG. 1, a data symbol vector Xk=[X0;k, . . . , XA−1;k]T taken from input data stream is encoded into a coded data symbol vector Ski=[S0;ki, . . . , SB−1;ki]T (i=1, 2, . . . , P) during B OFDM symbol periods by an STBC encoder 110. Here, k denotes a subcarrier index. The STBC-encoded data symbols are modulated by Inverse Fast Fourier Transforms (IFFTs) 1201, 1202, . . . , 120P corresponding to each transmit antennas 1301, 1302, . . . , 130P, and are then transmitted through corresponding transmit antennas.
The transmitted OFDM signals are received by the Q receive antennas 1401, 1402, . . . , 140Q, and are then demodulated into Ylj (j=1, 2, . . . , Q) by Fast Fourier Transforms (FFTs) 1501, 1502, . . . , 150Q corresponding to each of the receive antennas. The demodulated OFDM symbols are decoded by a decoder 160, thus determining transmitted data symbols.
The construction of the transmitter and the receiver in the STBC-OFDM communication system is described in detail with reference to FIGS. 2 and 3. Meanwhile, for convenience of understanding, the most basic configuration, i.e., a system having two transmit antennas and one receive antenna, is described. Systems having three or more transmit antennas and two or more receive antennas can be easily understood from the disclosure of the present invention.
FIG. 2 shows a schematic construction of an STBC-OFDM transmitter that implements STBC transmit diversity using two transmit antennas.
Referring to FIG. 2, an STBC-OFDM transmitter 200 includes a data source 205, a constellation mapper 210, a Serial-to-Parallel (S/P) converter 215, an STBC encoder 220, Inverse Fast Fourier Transformers (IFFTs) 225 and 225′, Digital-to-Analog (D/A) converters 230 and 230′, filters 235 and 235′, I/Q modulators 240 and 240′, up converters 245 and 245′ that convert frequency bands into Radio Frequency (RF) bands, amplifiers 250 and 250′, and transmit antennas 255 and 255′.
Data bits from the data source 205 are encoded into M-ary data symbols by the constellation mapper 210. The data symbols pass through an interleaver (not shown), are converted into parallel data symbols by the S/P converter 215, and are then input to the STBC encoder 220.
In the OFDM system using N sub-carriers, the vector of each input data symbol for a kth (k∈[0, 1, . . . , N−1]) sub-carrier can be defined as follows:Xk=[X0;k,X1;k]T  (1)where Xk is an M-ary data symbol having independent, identically distributed (i.i.d.) characteristics and (.)T is the transpose of a matrix.
The STBC encoder 220 receives the data symbol vector Xk=[X0;k, X1;k]T and generates a coded data symbol matrix Sk, which fulfill the above-described orthogonal design condition of Tarokh, to the plurality of the transmit antennas.
In the case of an STBC-OFDM communication system having a general code rate, for the kth sub-carrier, a data symbol vector Xk composed of A transmit data symbols passes through the STBC encoder, thus producing a two-dimensional data symbol matrix Sk having a B×P order, which corresponds to B OFDM symbol periods and P spaces, according to a predetermined code rate. Meanwhile, the STBC encoding method according to the predetermined code rate is known to those skilled in the art.
Accordingly, for convenience of description, for example, in the case where two transmit antennas (i=1, 2) are used, and a data symbol matrix Sk having a 2×2 order is produced by encoding two transmit data symbols, the data symbol vector Sk output from the STBC encoder can be expressed as the following Equation 2.
                              S          k                =                              [                                                                                S                                          0                      ;                      k                                        1                                                                                        S                                          0                      ;                      k                                        2                                                                                                                    S                                          1                      ;                      k                                        1                                                                                        S                                          1                      ;                      k                                        2                                                                        ]                    =                      [                                                                                X                                          0                      ;                      k                                                                                                            X                                          1                      ;                      k                                                                                                                                        -                                          X                                              1                        ;                        k                                            *                                                                                                            X                                          0                      ;                      k                                        *                                                                        ]                                              (        2        )            where the first and second columns of Sk represent data symbols for first and second transmit antennas, respectively. The first and second rows of Sk represent data symbols for first and second OFDM symbol periods, respectively. Also, (.)* represents a complex conjugate.
Furthermore, the coded data symbol matrix Sk encoded by the STBC encoder can be expressed as the following Equation 3 for N sub-channels.Sli=[Sl;0i,Sl;1i, . . . , Sl;N−1i]T  (3)where l=0 and 1, which represent first and second OFDM symbol periods antennas, respectively.
The data symbol Sl;ki encoded by the STBC encoder 220 is modulated into a base band by the IFFT 225. At this time, a guard interval longer than the period of the maximum delay spread of a channel is inserted between successive OFDM symbols in order to prevent Inter-Symbol Interference (ISI) due to a multi-path channel. Generally, a Cyclic Prefix (CP) is used as the guard interval so as to prevent the destruction of orthogonality that may occur due to the delay of sub-carriers. The signal is then transmitted to a wireless channel through the D/A Converters 230 and 230′, the filters 235 and 235′, the I/Q modulators 240 and 240′, the up converters 245 and 245′, the amplifiers 250 and 250′, and the antennas 255 and 255′.
For the more detailed construction of the STBC-OFDM transmitter, refer to “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency-Division Multiplexing” (L. J. Cimini, Jr., IEEE Trans. Commu., Vol. 33, No. 7, July, 1985; hereinafter referred to as “Reference 3”).
FIG. 3 is the schematic construction of an STBC-OFDM receiver 300 in the case where two transmit antennas and one receive antenna are used.
As shown in FIG. 3, an OFDM signal transmitted from an STBC-OFDM transmitter is received by a receive antenna 305, and is then input to an FFT 330 through an amplifier 310, a down converter 315, an I/Q demodulator 320 and an A/D converter 325.
The FFT 330 demodulates a digital signal output from an A/D converter 325 into a demodulated OFDM symbol Yl;m where m represents a sub-channel. Meanwhile, in the case where a receive diversity gain is provided by a plurality of receive antennas 305 and 305′, a linear combiner that combines demodulated OFDM symbols that are demodulated by a plurality of the FFTs corresponding to each of the antennas can be added. A decoder 335 then determines transmit data symbols by processing the demodulated OFDM symbol.
For example, in the case where one receive antenna is used, as shown in FIG. 3, if a CP is longer than the period of the maximum delay spread of a wireless channel and perfect synchronization is achieved at the receiver 300, Yl can be expressed as the following Equation 4 for N sub-channels.
                              Y          l                =                                            [                                                Y                                      l                    ;                    0                                                  ,                                                      Y                                          l                      ;                      1                                                        ⁢                                                                          ⁢                  …                                ⁢                                                                  ,                                  Y                                      l                    ;                                          N                      -                      1                                                                                  ]                        T                    =                                                    ∑                                  i                  =                  1                                2                            ⁢                                                H                  l                  i                                ⁢                                  S                  l                  i                                                      +                          W              l                                                          (        4        )            where Wl is Additive White Gaussian Noise (AWGN) having a size of N×1, its average is 0, and its distribution is σw2. Hli represents frequency response or transfer gain for the ith transmit antenna in the 1th symbol period. More detailed information is disclosed in “An Equalization Technique for OFDM Systems in Time-variant Multipath Channels,” W. G. Jeon, K. H. Chang, and Y. S. Cho, IEEE Trans. Commun., Vol. 47, No. 1, pp. 27-32, January, 1999; hereinafter referred to as “Reference 4”).
Meanwhile, the OFDM reception symbols Yl based on Equation 4 are expressed for an mth (m∈[0, 1, . . . , N−1]) sub-carrier as follows:Yl;m=Hl;mSl;m+Il;m+Wl;m Hl;m=[Hl;m1,Hl;m2]Sl;m=[Sl;m1,Sl;m2]T Il;m=Il;m1+Il;m2  (5)
In that case, Il;m serves as ICI, as disclosed in Reference 4.
When the Maximum Likelihood (ML) technique is employed so as to detect transmit data symbols from the received OFDM symbol Yl;m, optimal detection performance can be obtained. However, when the ML technique is applied as it is, problems arise in that complexity increases excessively and the amount of calculations increases exponentially in proportion to the degree of constellation of data symbols.
As an alternative for solving the above-described problems, the Alamouti technique, in which the ML method is simplified, may be used. When the number of transmit antennas is two or more, the Tarokh technique may be employed as disclosed in Reference 3. The above-described alternative method implements STBC decoding using simple linear calculation by assuming that there is no change in channel characteristics between two OFDM symbol periods, i.e., H0;mi=H1;mi.
For example, when STBC decoding is performed based on the Alamouti technique, Equation 6 is derived by simplifying calculation in such a way as to assume that H0;mi=H1;mi. Accordingly, decision variables R0;m, R1;m for determining the transmit OFDM symbol signal are calculated.
                              [                                                                      R                                      0                    ;                    m                                                                                                                        R                                      1                    ;                    m                                                                                ]                =                              [                                                                                H                                          0                      ;                      m                                                              1                      *                                                                                                            H                                          1                      ;                      m                                        2                                                                                                                    H                                          0                      ;                      m                                                              2                      *                                                                                                            -                                          H                                              1                        ;                        m                                            1                                                                                            ]                    ⁡                      [                                                                                Y                                          0                      ;                      m                                                                                                                                        Y                                          1                      ;                      m                                        *                                                                        ]                                              (        6        )            
Subsequently, transmit data symbols are determined by applying a predetermined symbol decision rule to the decision variables R0;m, R1;m calculated using Equation 6.
As described above, when the ML technique is applied to the detection of transmit data symbols in the conventional STBC-OFDM receiver (or STBC-OFDM signal detection device), a problem of excessive complexity arises. In order to solve this problem, decoding calculation is simplified by assuming that H0;mi=H1;mi.
However, in a time-variant channel environment in which a mobile terminal moves at high speed, there is a possibility that channel characteristics may vary between successive OFDM symbol periods because the degree of time-variation of a channel is high. Accordingly, when the conventional STBC-OFDM decoding technique is applied as it is in a time-variant channel environment, Co-Subchannel Interference (CSI) is generated due to the change in channel characteristics between successive OFDM symbols. That is, the influence of an error due to emi=H0;mi−H1;mi is included because it is assumed that H0;mi=H1;mi in Equation 6. As such, the influence is called CSI because it results from an interference signal based on the same sub-carrier of different transmit antennas.
The generation of CSI results in increased noise power, which causes the probability of making a decision error to increase. In particular, CSI increases in proportion to the number of transmit antennas. Since the increase in diversity gain is not proportional to the increase in the number of transmit antennas, the diversity gain is cancelled by CSI in a high-speed time-variant channel even though the number of antennas is increased to more than two. Thus, the effect thereof may be insignificant.
In the meantime, when the frequency responses of the individual sub-channels of a time-variant channel are estimated for individual OFDM symbol periods and the ML technique is applied as it is, the complexity of calculation increases, which may lead to complicated system configuration.